mu-tool: command not found

Maybe Your $PATH variable doesn't contain the default paths? echo $PATH should give something like:

[root@server ~]# echo $PATH
/usr/local/sbin:/usr/local/bin:/sbin:/bin:/usr/sbin:/usr/bin:/root/bin

If it's empty, You may set the PATH variable with this command

export PATH="/usr/local/sbin:/usr/local/bin:/sbin:/bin:/usr/sbin:/usr/bin:/root/bin"

On Debian possible default full paths are:

/usr/bin/mu-tool - from package: mailutils

You may install the required package with command apt-get install {package}

Error in other languages:
mu-tool: bevel nie gevind nie
mu-tool: comandă negăsită
mu-tool: comando não encontrado
mu-tool: commande introuvable
mu-tool: command not found
mu-tool: käsku ei ole
mu-tool: không tìm thấy lệnh
mu-tool: komanda nerasta
mu-tool: Kommando nicht gefunden.
mu-tool: kommandot finns inte
mu-tool: komut yok
mu-tool: nie znaleziono polecenia
mu-tool: níor aimsíodh an t-ordú
mu-tool: no se encontró la orden
mu-tool: no s'ha trobat l'ordre
mu-tool: opdracht niet gevonden
mu-tool: parancs nem található
mu-tool: perintah tidak ditemukan
mu-tool: príkaz nenájdený
mu-tool: příkaz nenalezen
mu-tool: команда не найдена
mu-tool: командата не е открита
mu-tool: 命令找不到
mu-tool: コマンドが見つかりません